As electronic devices become more and more portable and wireless, small and lightweight lithium ion secondary batteries with high energy density, are receiving attention as a power source for these devices. Lithium ion secondary batteries have a positive electrode comprising a lithium-containing transition metal oxide or the like, a negative electrode comprising a carbon material or the like, and a non-aqueous electrolyte.
In lithium ion secondary batteries, a separator is interposed between the positive electrode and the negative electrode to electronically insulate both electrodes from each other, and further to retain the electrolyte. As the separator, a microporous membrane mainly composed of a polyolefin such as polyethylene and polypropylene is used. The microporous membrane is formed by drawing a resin, generally.
However, such separator shrinks with heat under a comparatively low temperature of approximately 100° C. Therefore, a small short circuit may rapidly expand to cause a thermal runaway. That is, when a short circuit is caused by an intrusion of a foreign substance or a nail penetration test, the separator shrinks with the heat instantly generated. Based on such shrinkage, a damaged part of the separator becomes larger to expand the short circuit, causing the thermal runaway. Especially, when in an environment under a temperature over 150° C., the possibility for impairing the battery safety based on the shrinkage of a microporous membrane is high.
Thus, as schematically shown in FIG. 4, an attempt to make a paste electrolyte 40 to function as a separator. The paste electrolyte 40 includes a great amount of liquid electrolyte 41 containing a thickener, and filler particles 42 having electrically insulating property: The filler particles 42 function as a spacer between a positive electrode 43 and a negative electrode 44 (Japanese Laid-Open Patent Publication No. Hei 10-55718).
The paste electrolyte is a composite material of a liquid electrolyte with its viscosity increased by a thickener, and a filler having electrically insulating property. Therefore, the liquid electrolyte is sufficiently included in the paste electrolyte, and the paste electrolyte is excellent in securing lithium ion conductivity of a certain level. However, the paste electrolyte has a defect of impracticality, since its strength as a separator is insufficient.
Also, it has been proposed to use a porous film comprising a filler and a resin binder and being adhered on a surface of at least one of a positive electrode and a negative electrode as a separator (Japanese Laid-Open Patent Publication No. Hei 10-106530).
The porous film is formed by applying a raw material paste comprising a filler, and a resin binder dissolved in a solvent on a surface of an electrode plate and then drying it. Such paste includes fluorocarbon resin, polyolefin rein, or the like as a resin binder.
Further, in order to prevent an inducement of a battery internal short circuit caused by a separation of a part of an electrode material mixture from an electrode plate while manufacturing a battery, there has been proposed to use a porous film and a separator such as the above in combination (Japanese Laid-Open Patent Publication No. Hei 7-220759).
The porous films described in Japanese Laid-Open Patent Publication No. Hei 10-106530 and Japanese Laid-Open Patent Publication No. Hei 7-220759 are excellent to the extent that these can secure a certain level of strength and safety.
However, when a resin binder is dissolved in a solvent and then deposited on a surface of filler particles, as schematically shown in FIG. 5, the area of filler particles 52 covered by a resin binder 51 increases, which necessitates a usage of a great amount of resin binder. As a result, micropores among the filler particles decrease while the strength increases, and paths for an electrolyte or lithium ion between the positive electrode 53 and a negative electrode 54 tend to become insufficient. That is, it is difficult to secure sufficient lithium ion conductivity, while maintaining a certain level of strength.
Additionally, since a resin having suitable properties for a resin binder of porous film is not known, it is difficult to aim for a further improvement of the strength of the porous film, while maintaining lithium ion conductivity.